1. Field of the Invention
The invention relates to a low thermal conductivity heat barrier composition, a mechanical superalloy article provided with a protective heat barrier ceramic coating having such a composition, and a method of making the ceramic coating.
2. Summary of the Prior Art
For more than 30 years the makers of turbine engines for both land and aeronautical use have been tackling the problems of increasing turbomachine efficiency, reducing their specific fuel consumption, and reducing polluting emissions of the CO.sub.x, SO.sub.x and NO.sub.x type as well as unburned components. One way of dealing with these demands is to get close to the combustion stoichiometry of the fuel and thus to increase the temperature of the gases issuing from the combustion chamber and entering the first stages of the turbine.
As a result it has been necessary to make the materials used for turbines compatible with this increase in combustion gas temperatures. One solution adopted has been to improve turbine blade cooling techniques, which has led to a considerable increase in the technical complexity and the cost of production of the blades. Another solution has been to develop the refractory nature of the materials used in terms of peak working temperature, creep and fatigue life. This solution started to be used when superalloys with a nickel and/or cobalt base became available, and underwent a considerable technical advance as a result of the change from equiaxial superalloys to monocrystalline superalloys (giving a creep resistance improvement of 80 to 100.degree. C.). This route can now be exploited further only with substantial development costs, giving third-generation superalloys providing a further gain in creep resistance of approximately 20.degree. C. Beyond this a change in the family of materials is required.
An alternative to changing the family of materials is to deposit a heat-insulating ceramic coating, called a heat barrier, on the superalloy articles. This ceramic coating enables a cooled article to have, during continuous operation, a thermal gradient across the ceramic which is possibly more than 200.degree. C. The working temperature of the metal below is reduced in proportion, with considerable effect on the volume of cooling air required, the working life of the article, and the specific fuel consumption of the engine.
The ceramic coating may be deposited on the article to be coated by various processes, most of which belong to two different categories, namely sprayed coatings and physical vapour phase deposited coatings. Other deposition processes of the plasma assisted chemical vapour phase deposition (CVD) type can also be used.
For sprayed coatings a zirconia based oxide is deposited by a plasma spraying technique. The coating consists of a stack of melted ceramic droplets which are then quenched, flattened and stacked to form an incompletely densified deposit between 50 .mu.m and 1 mm thick. One of the characteristics of this kind of coating is a high intrinsic roughness (the roughness Ra being typically between 5 and 35 .mu.m). The commonest kind of degradation associated with this coating in use is the slow propagation of a crack in the ceramic parallel to the ceramic/metal interface.
The problem is rather different in the case of coatings deposited by physical vapour phase deposition. A deposition of this kind can be made by evaporation under electron bombardment. The main characteristic is that the coating consists of an assembly of very fine columns, typically between 0.2 and 10 .mu.m, which extend substantially perpendicularly to the surface to be coated. The thickness of such a coating can be between 20 and 600 .mu.m. Such an assembly has the useful property of faithfully reproducing the surface texture of the substrate covered. In particular, in the case of turbine blades this enables a final roughness of considerably less than 1 .mu.m to be obtained, which is very advantageous for the aerodynamic properties of the blade. Another consequence of the columnar structure of physical vapour phase ceramic depositions is that the gap between the columns helps the coating to deal very effectively with the compression stresses arising in operation due to the difference between expansion of the superalloy substrate and expansion of the coating. In this case long working lives under thermal fatigue at high-temperature can be achieved, and the coating tends to rupture in the region of the interface between the underlayer and the ceramic.
Chemical vapour phase deposition techniques produce coatings whose morphology is columnar and substantially equivalent to the morphology of physical vapour phase deposits. In both these techniques the formation of oxide results from a molecular reaction between metallic atoms or ions and oxygen.
The heat barrier coatings consist of a mixture of oxides, usually having a zirconia base. This oxide is a useful compromise between a material of fairly low thermal conductivity and a material of relatively high coefficient of expansion near that of the nickel or cobalt based alloys on which it is required to deposit the coating. One of the ceramic compositions which has proved very satisfactory is zirconia completely or partly stabilised by an oxide such as, for example, yttrium oxide: ZrO.sub.2 +6 to 8 Wt. % of Y.sub.2 O.sub.3. The yttrium oxide serves to stabilise the cubic allotropic variety C and/or the non-transformable tetragonal variety t' of the zirconia and thus to prevent martensitic phase transitions in response to excursions between ambient temperature and the high working temperature of the article.
The main requirement of a heat barrier coating is that it should slow down heat exchanges between an external environment of hot gases and the coated metal article, which is nearly always cooled by a forced flow of cold gases. Heat exchange between the ceramic coating and the metal below it can be by conduction and, to a lesser extent, by radiation. The thermal conductivity of an oxide is of course the sum of a phonic contribution which varies as a function of 1/T and of a radiant contribution which varies as a function of T.sup.3. In the case of partly or completely stabilised zirconias it is found that the radiant contribution is substantial beyond 500.degree. C. in a monocrystal (thermal conductivity increases rapidly with temperature) but is negligible up to 1200.degree. C. in a polycrystal since thermal conductivity is found to decrease as temperature rises. This phenomenon is due to retro-diffusion at faults in the polycrystal, such as grain junctions and porosities. In the case of heat barrier coatings which are polycrystalline in nature heat transmission by radiation is less than heat transmission by conduction. Consequently, to improve the heat insulation provided by a heat barrier the key controlling parameter is heat transmission by phonons.
There are several methods for decreasing the thermal conductivity of the coating, based on the fact that heat barrier coatings arc porous ceramic layers and the thermal conductivity of the coating is that of a heterogeneous assembly of two heat-conducting media, namely the ceramic material itself of intrinsic conductivity .lambda..sub.intr, and the pores or microcracks of the coating whose conductivity is close to that of the gas filling them under operating conditions.
The effective conductivity .lambda..sub.actual of the coating is between .lambda..sub.intr and the conductivity of air .lambda..sub.air. It can in fact be stated that .lambda..sub.actual is a complex function of .lambda..sub.intr and .lambda..sub.air and the morphology of the coating.
A first solution to the problem of obtaining a low thermal conductivity coating is to use a ceramic of conventional ceramic composition, such as zirconia partly stabilised by 6 to 8 wt. % of yttrium oxide, and to modify the morphology of the coating--i.e., the proporation, distribution and orientation of the pores and microcracks of the coating, or the arrangement of the material in the form of columns or strata--so as to reduce .lambda..sub.actual. This result can be achieved by modifying the coating deposition parameters.
A second solution is to try to reduce .lambda..sub.intr directly by modifying the chemical composition of the coating, without altering its morphology and while conserving the other properties of the coating. For example, it is well known that the introduction of yttrium into zirconia reduces its thermal conductivity by creating gaps in the lattice due to the zirconium ions having a different valency from the yttrium ions. In general the introduction of spot defects into the lattice which act as phonon retrodiffusion centres helps to reduce thermal conductivity, and this is the solution which is used in the present invention.
Published European patent application EP 0,825,271 A1 describes a ceramic coating having a zirconia base and containing two additional oxides, namely:
a first oxide serving to stabilise the tetragonal or cubic form of the zirconia, this oxide possibly being yttrium oxide, calcium oxide, magnesium oxide, indium oxide, scandium oxide, or ytterbium oxide; and, PA1 a second oxide serving to reduce phonic thermal conductivity and absorbing radiant energy in the waveband between 0.3 and 5 .mu.m to reduce heat conductivity due to photons. PA1 a first oxide serving to stabilise the tetragonal or cubic form of the zirconia, this oxide possibly being yttrium oxide, calcium oxide, magnesium oxide, indium oxide, scandium oxide, or ytterbium oxide; PA1 a second oxide serving to reduce phonic thermal conductivity; and PA1 a third oxide which absorbs the radiant energy in the waveband between 0.3 and 5 .mu.m to reduce thermal conductivity due to photons. PA1 depositing a bonding underlayer on the superalloy substrate; and PA1 depositing on the underlayer a ceramic coating comprising zirconia, a dysprosium oxide to stabilise the zirconia and reduce the phonic thermal conductivity of the zirconia, and, optionally, from 0 to 30 mol. % of an oxide selected from the group consisting of hafnium dioxide, cerium dioxide, uranium dioxide, and mixtures thereof.
In an alternative version the ceramic zirconia-based layer contains three additional oxides, namely:
However, the compositions proposed in this document are complex and expensive.